The present invention relates to combustion systems in gas turbine engines, and more particularly, to apparatus and systems related to the configuration and design of combustor nozzles and fuel injectors.
Combustion turbine engines or “gas turbines” are widely used in industrial and power generating applications. As will be appreciated, typical gas turbines includes an axial compressor positioned at forward end, a turbine positioned at an aft end, and one or more combustors about the middle portion of the engine. In operation, ambient air enters the compressor, and rotating blades and stationary vanes in the compressor progressively impart kinetic energy so to produce a supply of compressed air. From the compressor, the compressed air is directed into the combustor where it is mixed with a supply of fuel. The air/fuel mixture is then ignited and combusted within the combustor, and the resulting highly energized flow or “working fluid” is then expanded through the rotating blades of the turbine so work may be extracted therefrom. For example, the rotation induced by the flow through the turbine may rotate a shaft that connects to a generator so to produce electricity.
Certain types of combustor nozzles—commonly referred to as “micro-mixer nozzles”—include an array of mixing tubes about which a fuel plenum is formed. The supply of compressed air from the compressor is brought to a forward wall of the fuel plenum and the created pressure boundary drives the air through the tubes toward a combustion zone. The mixing tubes include fuel ports that fluidly communicate with the interior of the fuel plenum, and via these ports fuel is injected into the air moving through the tubes. Brought together in this manner, the fuel and air are suitably mixed before being expelled into the combustion zone for combustion.
Significant temperature differentials develop across different areas within the nozzle during operation. This is problematic because of the uneven thermal expansion that results and the stresses the uneven expansions causes. The temperature differentials develop for several reasons. First, as will be appreciated, the supply of air and fuel typically arrive at the nozzle at significantly different temperatures. Each flow also has different heat transfer characteristics due to the different properties and flow speed of each fluid. Second, the areas immediately surrounding the nozzle operate at different temperatures. For example, because the forward wall of the nozzle is positioned within the cap assembly, it is adjacent to a region having a much lower temperature than the aft portion of the nozzle, which borders the combustion zone. As a result, a significant consideration in designing micro-mixer nozzles relates to alleviating the temperature differentials that typically develop within the nozzle during operation. To the extend this can be achieved, the resulting stresses can be reduced and part life extended.
With conventional nozzle design, mixing tubes that pass through the interior of the fuel plenum usually reside at significantly lower temperatures than the outer walls that define the plenum. This is due to the lower temperature and heat transfer properties of the fuel. The outer walls are exposed to the higher temperatures that surround the plenum and, unlike the mixing tubes, do not have a fuel-air mixture flowing through a passageway defined through it. This results in the outer walls thermally expanding more than the mixing tubes and the development of high strain levels. It also will be appreciated that the conventional wall arrangement results in steep temperature gradients through the thickness of the wall. These conditions cause durability issues, lead to cracking and deformation, and reduce part life.
As will be appreciated, micro-mixer nozzle configurations results in a pressure drop across the nozzle, which is what drives the air through the mixing tubes at such high velocities. Such pressure losses, however, are parasitic and negatively impact overall system efficiency. A further objective of nozzle design is to minimize such losses while still achieving the benefits associated with these types of fuel injection systems. The pressure drop and the flow area through the nozzle defines the mass flow rate through the combustor. Another design constraint is the need to keep the diameter of the combustor head end small, which is due to cooling and packaging requirements. The combination of keeping the head end relatively small while still satisfying high mass flow rates makes the objective of maximizing flow area through the nozzle a significant one. The importance of this is further underscored by the fact that decreasing the cross-sectional area of the mixing tubes enhances the fuel-air mixing they provide. Thus, for a number of reasons, maximizing the area within the nozzle that can be dedicated toward mixing tube placement is important. It will be appreciated that to the extent these competing design objectives may be balanced more effectively, while still promoting performance, durability and cost-effectives, such improvements would be commercially demanded.